You Vanquish them. With either a potion or a spell, sometimes both. The higher level the demon the more powerful the potion has to be, the more powerful the spell.
Exceptions don't happen very much in my world. At least not when it comes to creatures and being switching sides. They're either Evil or they're Good. Sure they have their opposite counterparts, but usually they stay on the side they've always been on.
Walk inside of you? I can see what Sam meant by the exorcisms. I've handled tough before. But thanks for the warning.